1. Field of the Invention
The present invention relates in general to fuel sensor, and more particularly to fuel sensors of a type which sense the mixing ratio of methanol in a mixture of gasoline and methanol.
2. Description of the Prior Art
For dealing with the energy crisis and air polution problem, it has been proposed to use as the fuel of automotive internal combustion engine a mixed fuel, such as a mixture of gasoline and methanol. Japanese Patent First Provisional Publication 56-138431 shows one of fuel injection type engine systems operated on such mixed fuel. In the engine system of this publication, a measure is employed in which for practically operating the engine on the methanol-mixed gasoline, the fuel injection amount and ignition timing are controlled in accordance with the mixing ratio of methanol in the mixed fuel, which ratio is measured by a fuel sensor operatively disposed in a fuel container.
The fuel sensor is of a capacitance type which comprises a pair of spaced electrode plates submerged in the fuel. The dielectric constant of the fuel is detected by measuring the capacitance established between the electrode plates, and the mixing ratio of methanol is derived from the dielectric constant.
In operation, a DC voltage is applied between the electrode plate for electrostatic charging therebetween. Thus, one of the electrode plates serves as a positive electrode and the other serves as a negative electrode. The signal representative of the capacitance established between the positive and negative electrode plate under electrostatic charging is transmitted through lead wires to a separate control circuit where subsequent signal conversion and processing are carried out.
However, due to its inherent construction, the fuel sensor as mentioned hereinabove has the following drawbacks.
That is, due to application of DC voltage to the electrode plates, the plates are subjected to a corrosion in the methanol-mixed fuel.
In fact, even when the electrode plates are constructed of a corrosion-resistant plate, such as, stainless steel, nickel-plated metal, chrome-plated metal or the like, electrocorrosion can not be avoided. Furthermore, as a result of the electrodialysis, ionized metal in the fuel is attached to the negative electrode plate causing a non-negligible change in the capacitance characteristic of the electrode plates. This, of course, induces a considerable error in measuring the mixing ratio of methanol in the mixed-fuel.
In addition to the above, the corrosion products in the fuel tend to clog the fuel piping of the engine system.